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rachelbiscoe
the ratio of used books to new books is 9:4 because 13-4=9 and the ratio of all books to used books is 13:4 because there are 13 total books and 4 used books
Flowerly
Hey there!

13 shelfs, and 4 are used . . .

13 - 4 = 9

This means that 9 of the books are used.

9 + 4 = 13

The ratio is 9:4 <-----------

Hope this helps you.
Have a great day!